Clarification of the circulatory patho-physiology of anaesthesia – Implications for high-risk surgical patients

Abstract

The paper examines the effects of anaesthesia on circulatory physiology and their implications regarding improvement in perioperative anaesthetic management. Changes to current anaesthetic practice, recommended recently, such as the use of flow monitoring in high risk patients, are already beginning to have an impact in reducing complications but not mortality [1]. Better understanding of the patho-physiology should help improve management even further. Analysis of selected individual clinical trials has been used to illustrate particular areas of patho-physiology and how changes in practice have improved outcome. There is physiological support for the importance of achieving an appropriate rate of oxygen delivery (DO2), particularly following induction of anaesthesia. It is suggested that ensuring adequate DO2 during anaesthesia will avoid development of oxygen debt and hence obviate the need to induce a high, compensatory, DO2 in the post-operative period. In contrast to the usual assumptions underlying strategies requiring a global increase in blood flow [1] by a stroke volume near maximization strategy, blood flow control actually resides entirely at the tissues not at the heart. This is important as the starting point for understanding failed circulatory control as indicated by 'volume dependency'. Local adjustments in blood flow at each individual organ - auto-regulation - normally ensure the appropriate local rate of oxygen supply, i.e. local DO2. Inadequate blood volume leads to impairment of the regulation of blood flow, particularly in the individual tissues with least capable auto-regulatory capability. As demonstrated by many studies, inadequate blood flow first occurs in the gut, brain and kidney. The inadequate blood volume which occurs with induction of anaesthesia is not due to blood volume loss, but probably results from redistribution due to veno-dilation. The increase in venous capacity renders the existing blood volume inadequate to maintain venous return and pre-load. Blood volume shifted to the veins will, necessarily, also reduce the arterial volume. As a result stroke volume and cardiac output fall below normal with little or no change in peripheral resistance. The resulting pre-load dependency is often successfully treated with colloid infusion and, in some studies, 'inotropic' agents, particularly in the immediate post-operative phase. Treatment during the earliest stage of anaesthesia can avoid the build up of oxygen debt and may be supplemented by drugs which maintain or restore venous tone, such as phenylephrine; an alternative to volume expansion. Interpretation of circulatory patho-physiology during anaesthesia confirms the need to sustain appropriate oxygen delivery. It also supports reduction or even elimination of supplementary crystalloid maintenance infusion, supposedly to replace the "mythical" third space loss. As a rational evidence base for future research it should allow for further improvements in anaesthetic management.

Assessment of Haemodynamic Response to Induction of General Anaesthesia in Healthy Adult Patients Undergoing Elective Orthopaedic Surgery by Using a Continuous Non-invasive Cardiovascular Monitoring

“…We feel that perhaps doing so without fully knowing an individual patient's physiology could potentially lead to increased perioperative complications. This is supported by work done by Wolff et al which suggests that maintaining cardiac output at, or close to, levels as measured prior to induction is likely to be a good reflection of adequate cardiac output and oxygen delivery [13].…”

Well, here’s another nice mess you’ve gotten me into!

“…(28) This has important implications for surgical fluid management. (22) Excess fluids and Na+ to replace non-existent "third space loss" are not only unnecessary (15) but may actually damage the EGL directly by compression and distortion and also lead to an increase in atrial natriuretic peptide (ANP) which itself damages the layer. The implication is that "third space loss" may actually be precipitated by excess fluids!…”

Multimodal monitoring (MMM) in the perioperative period

“…Experiments using propofol in dogs showed that it produced an increase in venous capacitance due to venodilation (not peripheral arteriolar vasodilation), which we feel a key feature of anaesthesia that has been very little recognised. (18,19) Following venodilation the fall in MAP is due not only to falling preload, SV and CO with a consequent reduction in oxygen delivery (DO2) but also due to a shift of volume out of the arterial tree into the dilated venous compartment. (19) This mimics hypovolaemia.…”

“…(18,19) Following venodilation the fall in MAP is due not only to falling preload, SV and CO with a consequent reduction in oxygen delivery (DO2) but also due to a shift of volume out of the arterial tree into the dilated venous compartment. (19) This mimics hypovolaemia. The question is whether we should maintain venous capacitance with liberal fluids or by administration of a venoconstrictor?…”